The goal of the proposed research is twofold: to elucidate the molecular basis of membrane excitation and to define the molecular events involved in the regulation of membrane excitability by cell-cell interaction. The work will be carried out using cloned tissue culture lines of nerve and muscle cells. A variety of animal toxins will be used to identify and localize in vivo, membrane components involved in the excitation process, to isolate these components in pure form, and to aid in the selection of mutant cell lines defective in some step in the excitation process. Membrane components involved in excitation will be identified by binding toxin, which will in turn be localized by its radioactivity, in the case of labeled toxin, or by its ability to bind ferritin or fluorescein labeled anti-toxin antibody. Mutant cell lines unable to bind a particular toxin, and hence defective in a particular membrane component, will be selected using the cytotoxic killing that results from binding of antibody and complement to a membrane antigen. In these experiments the membrane antigen will be the respective membrane-bound toxin. The mutant cell lines will in turn be used to investigate the excitation process and to define the molecular basis for the trophic effect of nerve cells on muscle cells. In the latter case the requirement for a particular membrane component for innervation will be investigated using a variety of mutant cell lines. The effect of innervation on the synthesis and function of particular membrane components will be investigated.